This invention relates to packed-column supercritical fluid chromatography (SFC) and improvements therein.
SFC has been used in chromatographic analyses for many years. See for example T. H. Gouw et al., J. Chrom., Vol. 68, pages 303-323 (1972) using SFC in packed columns. It has also long been known to carry out SFC analyses in open-tubular or capillary columns as described in Novotny et al., U.S. Pat. No. 4,479,380.
SFC is a generic name for a type of chromatography using mobile phases that are dense gases. In SFC, the mobile phase is a fluid subjected to temperatures and pressures generally near its critical point. Fluids at those conditions have densities much closer to liquids but often exhibit greater solute diffusion characteristics than liquids. In SFC, the solvent strength changes with density and pressure drops create density and retention gradients.
Gas chromatography (GC) is in general known to exhibit very high resolving power which enables the analysis of complex materials of volatile compounds. However, only generally stable, volatile compounds can conveniently be analyzed by GC and it is not ordinarily easy to dramatically change selectivity. On occasions, some samples have been subjected to chemical derivatization to allow the use of GC. This, however, is not always a complete solution. Some compounds react more fully than others and this variable conversion rate may make the result less certain. Multiple reactions might be required to get all the solutes in a sample to be volatile and stable enough for GC. Those solutes separated, however, are not the original solutes in the sample thereby further decreasing certainty in the results.
Liquid chromatography (LC) is known to enable the analyses of labile and relatively nonvolatile compounds, but it is generally regarded to be a low resolution analytical method. There are relatively few selective or sensitive detectors that work well with the liquid mobile phases typically used in LC. As a consequence, selectivity adjustment is often used in place of efficiency to resolve components in a mixture. Often, complex samples are not easily resolved by LC. It may require a complex sample to be split into multiple parts with each part undergoing different pretreatments and analytical methods. This can be expensive and time-consuming.
SFC is often regarded as being intermediate between GC and LC. In general, packed-column SFC is superior to open-tubular SFC in achieving shorter analysis times; however, open-tubular SFC columns may provide a higher number of theoretical plates at the same pressure drop.
Many have believed that the maximum permissible pressure drop may determine when packed columns may be used for rapid analysis and when capillary columns are needed for high efficiency separations. See, for example, Novotny et al., U.S. Pat. No. 4,479,380. It has also been believed that when pressure drop across the column becomes too high it may lead to increased capacity factors and therefore to broader peaks. See Mourier et al., Chromatographia Vol. 23 No. 1 Jan. 1987 pp. 21-25. In an article by Schoenemakers et al., Chromatographic Vol. 24, pp. 51-57, 1987, the effects of column pressure drop using packed SFC columns is discussed. A plot in FIG. 1 of theoretical plate efficiency [N] versus pressure drop for three tests suggests that efficiency is at a maximum of about 25,000 plates at about 20 bar and decreases gradually at higher pressure drops above about 25 bar.
Gere, Anal. Chem. 54, 736-740 (1982), reported the use of SFC in small particle diameter packed columns at high pressure drop, for example, 184 bar in FIG. 3. However, at that pressure drop, the column efficiency was reported to be 18,750 theoretical plates.